Photoglow tube



D. D. KNOWLES PHOTOGLOW TUBE April 17, 1934.

Filed Jan. 18, 1929 lNVENTOR fien/e fl Know/e3.

ATTORNEY Patented Apr. 17, 1934 UNITED STATES ATENT OFFICE PHOTOGLOWTUBE Dewey D. Knowles, Wilkinsburg, Pa, assignor to WestinghouseElectric & Manufacturing Company, a corporation of PennsylvaniaApplication January 18, 1929, Serial No. 333,389

6 Claims.

' shall maintain its electrical characteristics substantially invariablethroughout long periods of operation.

A further object of my invention is to provide an electrical-dischargedevice of the character described above in which the range of wavelength of radiation to which it is responsive may be varied between widelimits.

In connection with many branches of the electrical art, it is desirableto provide a relay device adapted to initiate and cut off the flow ofcurrent of considerable magnitude in response to the amount ofillumination falling upon a particular point or area. The presentinvention provides a single electrical-discharge tube having one of itselectrodes covered by a photo-electrica11y-sensitive substance. When avoltage is impressed between the electrodes of such a tube, a criticalvoltage exists below which substantially negligible current flowsthrough the tube and above which a current of the magnitude of one-tenthof an ampere is conducted in connection with a glow in the tube, and thevalue of this critical voltage decreases as the intensity of radiationfalling upon the photo-electrica1ly-sensitive electrode in 0 creases.

It is true that a number of substances are known which arephoto-electrically responsive and will, accordingly, vary theelectrical-current flow through a gas by which they are surrounded inaccordance with the amount of illumination falling upon their surfaces.However, in the case of substances heretofore known, it is found thatthe sensitivity of the substance to light varies with time, perhaps dueto a chemical combination between the photo-electric substance and thegaseous atmosphere. In accordance with one feature of my presentinvention, I have found that, if the photo-sensitive surface comprisesan underlying layer of silver or, alternatively, an alkaline-earthmetal, or preferably magnesium,

which has superposed upon it an extremely thin layer of an alkali metal,such as caesium, the electrical characteristics of a gaseous dischargepath comprising one of the noble gases, such as,

helium, remained unaltered through extremelylong periods of use.Light-sensitive surfaces of the character above described have thefurther advantage of being capable of simple and quick manufacture byprocesses well adapted to largescale production.

7 With the foregoing principles and objects in mind, my invention may bebetter understood by reading the following specification, taken inconnectionwith the drawing in whichthe single figure H is a View, partlyin elevation and partly a longitudinal section, of a tube embodying theprinciples of my invention.

Referring in detail to the drawing, the tube comprises a vacuum-typecontainer 1 of ordinary form into which are sealed a cathode lead 2andan anode wire 3 of the general form shown in the drawing. The lead 2lies parallel, and closely adjacent, to the wall of the container 1 fora considerable distance, its upper end being bent and I wire 3 but isspaced away from it by a distance 7 of the order of magnitude of thediameter of wire 3.

A tube, constructed as hereinbeiore described, is first exhausted bymethods well known in the vacuum-tube art. In accordance with suchmethods, the tube is heated to a temperature as high as the glass wallthereof will withstand without collapsing, but before sealing off, anoble gas, at less than atmospheric pressure, is introduced into thetube by any method well known in the art. It may then be sealed off fromthe vacuum pump. Such having been done, a highfrequency magnetic fieldis produced in the vicinity of the electrode l, and the latter is heatedto such temperature that the magnesium 5 thereon is vaporized andimmediately deposited on the walls of the tube, making contact with thewire 2 and forming the base for the photo-sensitive film. At the sametime, the magnesium acts as a getter for any gases or vapors remainingor liberated during the sealing-off process. The sleeve metal.

6 prevents the vapor of magnesium from depositing on the glass near thejoint with lead 3 and thereby insures the insulation of the latter fromthe magnesium coating.

It will be noted that, while I have described magnesium as the metal tobe vaporized, metals of the alkaline-earth group and even aluminumalloys, such as the well known calcium-magnesium alloy misch metal, maybe utilized for this purpose. The metal to be used depends uponconvenience of manufacture and the particular characteristics of thephoto-electric tube which are desired.

Where it is desired to employ a base layer of silver, the latter may beformed by subliming silver from an electrically'heated wire within thecontainer or even by the chemical-reduction methods well known in themirror-making art.

In order to provide the other element of the pair of metals which arecombined in the photoelectric surface which I have invented, severalmethods and materials are available.

vance with one method, a small amount of theva- In accordpol of a metalor metals may be introduced,-after heating a small tube containing thedesired The metals chosen should be suchas have a maximum on theiremissivity versus wavelength curve in the region of the wave length ofthe radiation to which response is desired. .Usuallyresponse is desiredto radiationin the yisiblespectrum; hence, 1ithium,.sodium, po-

tassum, rubidium and caesium, which have maxima of emissivity in thevisible region, are ordinarily preferable for use.

A way which Iprefenhowever, consistsin painting a weak solution of analkali-metal trinitride, such as caesium trinitride, on the portion .ofthe electrode 4 considerably removed from the magnesium 5.

Stillanother method of settingfree therequired alkali metal istoplace asmall amount of some salt thereof,which will not yield a deposit whenheated in vacuum to about 500 degrees C., on an electrode within thetube and produce a discharge which shall bombard the salt with ions and,consequently, decompose it.

After the vaporization of the .magnesium, it

:may, in general, be necessary to heat a small portion of the tubewallto clear a window so that light may enter and strike. upon thephoto-sensitive surface.

In the foregoing manner, a layer of. magnesium 7 will be deposited-onthe tube wall and, upon the surface of the latter, will be deposited athin layer of caesium 8. It will be recognized thatv the proportions ofthe layers '7 and 8 shownin the drawing are diagrammatical only and donot represent the true magnitude or relative proportion ductivitythereof that ionization by collision will increase to an enormousdegree, and a very considerable current will flow between the electrodesof tube 1 accompanied by a glow in. the e.

flow, once initiated, will persist even after the illumination of thetube ceases. hand, if the voltage impressed upon the tube is On theother periodic, falling to zero at intervals, current flow onceinitiated will persist until the voltage falls to zero for the firsttime after the illumination is cut off. The gas in the tube will thenalmost instantaneously return to its substantially nonconductivecondition, and appreciable current flow will only begin again whenradiation is again incident to the light-sensitive surface.

In accordance with my invention, I have found that the electricalcharacteristics of the tube, such as the critical voltage fora givenintensity of illumination, remain substantially unaltered throughout anactive life of hundreds of hours;

Such a permanency of electrical characteristics is obviously of thehighest importance from a practical standpoint and is found in the caseof no other gas-filled photo-sensitive discharge tubes which carrycurrents of the magnitude of 1105 one-tenth of an ampere, such as'theabove-described device is capable of carrying. Such a performance is tobe distinguished from that of ordinary photo-electric tubesinwhich theourin 'micro-amperes. That such relatively large current flow should notcause variation in the electrical characteristics of the photo-sensitivedevice is a very remarkable fact, and by obviating the employment ofauxiliary amplifying devices;

produces a device of high commercial value.

Another importantpractical advantage of the device above describedlies-in the fact that, by

.properly choosing the base layer 7 and the surface layer 8, theparticular wave length of radiation to which the device is mostsensitive may -be Varied over a wide range; thus, by employing barium asthe outer layer, in combination with the magnesium base, a device havingmaximum sensitivity in the visible region of the spectrum.

is obtainable. By substituting caesium for barium, the maximumsensitivity is displaced toward the blue-green region of the visiblespectrum. The-substituttion of rubidium, potassium and sodium as thesurface layer successivelydecreases the wave length of maximumsensitivity farther and farther into the ultra-violet. On the otherhand, by employing two or more of the alkali *metals, in combination, inthe surface layer,sen-

sitivity corresponding to that of both components is obtained, so that asubstantially uniform sensitivity over a very considerable range of wavelengths may be attained. In fact, by properly combining andproportioning the various substances enumerated as suitable for thephotosensitive layers, a wide range of photo-electric characteristicsmay be obtained.

While I have described the foregoing particular embodiments of myinvention, it will be evident to those skilled in the art that manymodifications thereof may be employed to meet special problems withoutdeparting from the broad principles which I have described. Accordingly,I

desire that the claims shall begiven the broadest rents actuallytraversing the device are measuredino reasonable interpretation to whichtheir terms are susceptible in view of the prior art.

I claim as my invention:

1. A glow tube comprising a photo-sensitive electrode having an outerlayer of a metal of the alkali metal group and an underlying layer of ametal of the alkaline-earth group surrounded by an atmosphere adapted toglow in operation comprising a noble gas at less than atmosphericpressure and another electrode cooperating with said first mentionedelectrode.

2. A glow tube comprising a photo-sensitive electrode having a layer ofametal of the alkali metal group and an underlying layer of a metal ofthe alkaline-earth group surrounded by an atmosphere adapted to glow inoperation comprising a noble gas at less than atmospheric pressure andanother electrode cooperating with said first mentioned electrode.

3. A glow tube comprising an anode having a relatively small exposedsurface and a photosensitive cathode comprising an outer layer of ametal of the alkali group and an underlying layer of a metal of thealkaline-earth group, said electrodes being surrounded by a noble gas ata substantial pressure adapted to glow in operation.

4. A glow tube comprising an anode having a relatively small exposedsurface and a photosensitive cathode comprising a layer of a metal ofthe alkali group and an underlying layer of a metal of thealkaline-earth group, said electrodes being surrounded by a noble gas ata substantial pressure adapted to glow in operation.

5. A glow tube comprising an anode having a relatively small freesurface, a gaseous atmosphere at a substantial pressure adapted to glowin operation and a photo-sensitive cathode having a relatively extensivesurface and comprising an underlying layer of a metal of thealkalineearth group and an outer layer of a metal of the alkali group.

6. A glow tube comprising an anode having a relatively small freesurface, an atmosphere of a gas at a substantial pressure adapted toglow in operation and a photo-sensitive cathode having a surface area atleast ten times the free surface of said anode and comprising anunderlying layer of magnesium and an outer layer of caesium.

DEWEY D. KNOWLES.

